Ion Sources

RF Charge Exchange Ion Source

The purpose of the ion source
is to produce either positive or negative ions from neutral atoms. Positive
ion sources are placed inside the tank of a single-ended accelerator; negative
ion sources inject the ion beam into the tank of a tandem accelerator. Different
types of sources may be used depending on the mass and charge of the desired
ion.

An RF (radio frequency) ion source produces positive ions. A gas
or gas mixture is bled into a quartz bottle; an RF oscillator connected to the
quartz bottle dissociates the neutral gas. A voltage difference (usually
about 2-6 kV) is used to push the ions out of the chamber through the exit
aperture, making a continuous beam. To produce a negative beam, the positive
beam is immediately injected into a rubidium charge exchange cell.

A Duoplasmatron ion source can produce either a positive or a negative
ion beam depending on the placement of the intermediate (zwischen) electrode.

SNICS-II Ion SourceThe Source of Negative Ions by Cesium Sputtering
(SNICS) produces a negative ion beam. Cesium vapor comes from the cesium
oven into an enclosed area between the cooled cathode and the heated ionizing surface.
Some of the cesium condenses on the front of the cathode and some of the
cesium is ionized by the hot surface. The ionized cesium accelerates towards
the cathode, sputtering particles from the cathode through the condensed
cesium layer. Some materials will preferentially sputter neutral or positive
particles which pick up electrons as they pass through the condensed cesium
layer, producing negative ions.

The Pelletron Accelerator

The principal founder of the National Electrostatics Corp., Professor Raymond G. Herb,
was the first person to make the Van de Graaff electrostatic generator practical for use
as an MeV ion beam accelerator in the early 1930's. The basic concepts of the pressurized
generator with a reliable charging system, positive ion source integrated into the high
voltage terminal, graded accelerating tube and column were developed by Prof. Herb, his
students and colleagues at the University of Wisconsin.

In the early 1960's, the basic Pelletron inductive charging
system was invented by Dr. James A. Ferry (President of National Electrostatics) in
association with Prof. Herb. In 1965, with a reliable Pelletron charging system and
the unique NEC all metal and ceramic accelerating tube well in hand, the decision was
made to form National Electrostatics to produce the Pelletron accelerators.

NEC Pelletrons, whether S-series or U-series, come in two
configurations, tandem and single-ended. In both cases, ion beams (both positive and
negative) are accelerated by an electrostatic field, i.e. a potential
drop, resulting in a true DC beam.

In single-ended machines, one end has the high-voltage terminal
containing an ion (e.g. see Positive Ion Beam Sources) or
electron source, and the other end is at ground,
where the ion/electron beam emerges from the Pelletron with an energy
roughly equivalent to the terminal voltage. Reversible polarity, single-
ended Pelletrons are available for producing positive ions or electron
beams from a single system.

By contrast, tandem accelerators have both ends at ground with the high-
voltage terminal in the middle. Tandems are always charged positively,
so that the negatively-charged injected ions (e.g. see
Negative Ion Beam Sources)
accelerate to the terminal. Inside the terminal is a stripper, which uses a gas
(usually N2 or Ar) or a thin carbon foil to collisionally remove electrons from
the incoming negative ions. The now positively-charged ions experience a second boost
of acceleration (hence the name "tandem" accelerator) as they exit the
terminal and travel down the acceleration tube to ground at the high-
energy end of the machine. The primary benefits of the tandem
configuration are:

Much higher beam energy for a given terminal
voltage -- in most cases the final beam energy is approximately (q + 1)
times the terminal voltage, where q is the charge state to which the ions
are stripped in the terminal.

With the ion source(s) external to
the pressure vessel, maintenance requiring entry into the tank and letting
the acceleration tubes up to atmosphere is minimized.